The development of RFID (radio frequency identification) systems has made great progress over recent years so that many types of RFID systems have reached readiness for big installations. A conventional RFID system in general comprises a reader and a plurality of wireless data carriers so called transponders or tags 2. Each tag comprises at least one non-volatile memory, which usually is an E2-memory such as an EEPROM. The reader and the plurality of tags communicate with each other through antenna means 10 using RF-signals. The reader can read and write data from/to the memory of the tags.
Such RFID systems can be used to keep track on articles provided with such a tag during a manufacturing process or for warehousing management. However, there are many other applications for RFID systems of the above-mentioned kind in access systems like car access or control systems. In such applications, very often it is not only sufficient to read the data stored in the non-volatile memory of the tag, but also to write data into its memory.
In FIG. 1, a conventional method of operating a conventional RFID system is shown. In a first step S1 the reader 1 identifies a tag 2, which is in his reception range by sending an identification command IDENTIFY. In this case, for example, tag 1 is in the reception range of reader and, in step S2, tag 1 receives and decodes the identification command IDENTIFY of the reader and sends back the ID number ID NR1 stored in his memory. After identification of step S1 and the receipt of the ID number of tag 1 ID NR1 in step S2, the reader, in step S3, sends a write command WRITE TAG1 to tag 1 to write data into the non-volatile memory of tag 1, which as a first wireless data carrier. After the data which are included in the write command WRITE TAG1 or have been transmitted before have been written into the EEPROM of tag 1, tag 1, in step S4, sends back a write acknowledge signal for confirming that the writing into the memory was successful.
However, storing of data into a non-volatile E2-memory such as an EEPROM takes a considerable amount of a time (e.g., 10 to 20 ms or even more). The time needed for a writing operation of an EEPROM is therefore considerably longer than for a reading operation (about 10 to 100 times). Approximately the same amount of time is needed for encrypting or decrypting data or commands with an encryption/decryption element of the tag or for reading data from a sensor connected or part of the tag. Therefore, a non-volatile memory and a encryption/decryption element and a sensor for instance are slow response elements. With conventional methods of operating a RFID system, the reader waits until the tag processed data with the slow response element and responses. Therefore, the reader for instance waits until the writing operation into the EEPROM is completed by receiving a write acknowledge signal from the tag and does not perform any communication with other tags which are presently available in the reception range of the reader. Therefore, these RFID systems comprise for instance the disadvantage that tags which pass during the time of writing operation or encryption or decryption operation or reading data from a sensor of one tag can not be identified or read out and can therefore pass the reader unnoticed. A further disadvantage is that the available communication capacity is not sufficiently used due the inactivity of the reader during to a writing operation of a tag.